The present invention relates to a method and an arrangement in accordance with the preambles to patent claims 1 and 6 respectively.
When driving a heavy vehicle, such as a truck or a bus, provided with a mechanical gearbox and a mechanical clutch, e.g. of disc type, changing down to lower gears is particularly problematical. Depressing the clutch pedal at the time of gear changing results in the engine speed decreasing quickly when the accelerator pedal is released, partly because of the internal friction in the engine. A large number of successive gear changes further reduces the engine speed to idling speed.
Consequently, after the gear change has taken place, the engine has to be speeded up again, which is typically accomplished by the driver allowing the clutch pedal to rise so that the engine is speeded up by being connected to the driveline. Engine speed adjustment is thus brought about via the sliding connection with the gearbox, the final gear and the drive wheels.
A disadvantage of the state of the art is that the sometimes large speed differences and masses involved in such speed adjustment cause considerable clutch wear, even in normal gear changes during normal driving, owing to the aforesaid engine speed reduction when gear changing is initiated. In addition, there is loading of all the other parts of the driveline and inducing of oscillations in the driveline. These result in inferior comfort because of disturbing noise and physical vibration in the vehicle, which is particularly troublesome if the vehicle is a bus.
One object of the present invention is to eliminate the problems of the state of the art. This is achieved in a method and an arrangement of the kind mentioned in the introduction by means of the features indicated in the characterising parts of patent claims 1 and 6 respectively.
It entails using simple means for preventing the undesirable drop in engine speed at the time of clutch disengagement, thereby enabling substantial reduction in clutch loading and wear and in the corresponding loading of other driveline components. Further advantages which may be indicated include simplified driving of a vehicle which is equipped in accordance with the invention, and increased comfort, which is particularly advantageous in buses but is also significant for other types of heavy vehicles. Equipping the vehicle additionally with a hydrodynamic supplementary brake, a so-called retarder, connected to the gearbox results in further advantages in that the retarder cooling, shared with the engine cooling, is maintained at a higher level by means of an arrangement according to the invention than would otherwise be the case upon engine speed reduction. A system according to the invention thus prevents the risk of coolant boiling in the retarder and makes it possible to dimension the cooling arrangement so that the latter need not operate with the excessive and almost momentary additional heat which may otherwise occur on completion of a gear change procedure. It should be mentioned here that in normal driving it is in principle never, or at least extremely rarely, desirable for the engine speed to be reduced to idling speed.
Regarding the indication of the clutch being disengaged, it may be stated that sensors for emitting such signals are already known but are conventionally designed, for example, to communicate with a road speed maintainer and/or to make changeover possible in any split gearbox fitted. Such conventional sensors may nevertheless be used in the invention.
The invention is applicable both in purely mechanical gearboxes with mechanical linkages and in servo-assisted gearboxes. Relevant gearboxes include a main gearbox and also, where applicable, a split and/or range gearbox or the like.
Engine speed control systems are already known and will not be described here.
Advanced control systems have recently been developed which adapt the engine speed to the input speed of a gearbox unit on the basis of a number of sensors situated at various points in the driveline, but these systems are applied in such a way that the clutch is not used in normal driving and the speed adjustment is designed to be so accurate that a gear changeover can be initiated directly without operating the clutch. Such sophisticated control systems are relatively expensive, however, since they require a more complex structure as regards both the components involved and the software programming of the control system. With the system according to the present invention, engine speed adjustment or maintenance is initiated by the driver operating an entirely manual or a servo-assisted clutch, but the clutch is otherwise used in a conventional manner. The advantage of the present invention is thus that it avoids the speed decrease which occurs with conventional systems, and hence also avoids the problems associated with the latter. The invention may thus be used at relatively low cost and in a simple manner without the control system becoming complex.
The driving data which may require observation and recording include acceleration, vehicle speed, engine speed, power mobilisation and torque extraction. Depending on the application, the system may be designed to observe some or all of these driving parameters to provide the basis for estimating, when the driver disengages the clutch, an engine speed which is at least in the vicinity of the actual engine speed after the gear change has taken place.
Further advantages are achieved by the features in the other patent claims and will be described below.